This invention relates to a process and an apparatus for conducting the hot gas in the manufacture of filaments by the dry spinning process, in which the hot gas catches the filaments immediately under the spinning nozzle.
The spinning chambers used for the production of filaments by the dry spinning process comprise two operational regions, namely the drop-chamber, which is usually circular in cross-section, and the spinning head mounted on the drop-chamber. The cross-section and length of the drop-chamber depend on the rate of throughput of polymer, the quantity of solvent and quantity of hot gas used for evaporating the solvent from the filaments. The drop-chamber is generally heated by a heating jacket at temperatures which are usually above the boiling point of the solvent.
The hot gas used may be air, but an inert gas is preferred. The hot gas acting on the filaments generally flows in the same direction as the filaments. The quantity and temperature of the gas depend on the nature of the polymer and of the solvent and on the spinning output, and are generally required to be determined experimentally on the basis of required quality of the given spinning material. Temperatures in the region of 150.degree. to 400.degree. C. are customarily employed.
The spinning head contains the assembly of a spinning nozzle, an apparatus for supplying and distributing the spinning solution, and filtering elements. The number of spinning apertures and their diameter and form depend on the nature of the polymer solution to be spun and the spinning output.
The nozzles used in the present invention are ring spinning nozzles in which the nozzle apertures are arranged in a circular ring. Pot nozzles are also known which have a circular surface with the nozzle apertures arranged either over the whole of this surface or on a peripheral circular ring of the surface.
The spinning head also contains devices for introducing the hot gas into the drop-chamber. In ring spinning nozzles, it is known to let the hot gas stream out both inside and outside of the ring.
The spinning nozzle is regulated so that the temperature of the spinning solution in it is noticeably below the evaporation temperature of the solvent. The temperature of the hot gas, on the other hand, must be higher than the evaporation temperature so that evaporation of the solvent may begin immediately beneath the spinning nozzle.
When the hot gas is inside the spinning head, and indeed above the spinning die, it generally flows axially. The velocity of the gas is so low that only a minimal pressure loss occurs across the metal gauges closing the air flow regions off from the drop-chamber. This pressure loss is not sufficient to have any significant equalizing effect on turbulence and temperature differences. The flow velocities are so low that sufficient mixing does not occur.
If the gas flow has not been balanced, it causes unsteady flow underneath the spinning nozzle, so that the extruded monofilaments which still contain large quantities of solvent undergo transverse movements which lead to fluctuations in the titre of the individual filaments and to filaments sticking together. Such spinning defects cause problems in the subsequent after-treatment of the filaments and the yarn manufacturing process and are therefore undesirable.
It is known to improve the equalization of the gas flow by feeding in the gas above the spinning nozzle radially or tangentially at a high velocity transversely to the general direction of the filaments and producing an annular rotation flow (U.S. Pat. Nos. 2,615,198, 3,509,244, 3,111,368 and 4,123,208). This requires the use of a pot nozzle since a rotational flow extending to the center produces a high vacuum at the center which would be, however, covered by the spinning nozzle. A pot nozzle has, however, the fundamental disadvantage that, even when the spinning apertures are arranged in a circular ring, the filaments situated more centrally are not so easily reached by the hot gas and are therefore not sufficiently dried. If a ring nozzle which is open at the center were used, on the other hand, the vacuum produced with the rotational flow would cause severe backflow within the set of filaments and would therefore also lead to spinning defects.
In order to obtain equal drying of the internally and externally situated filaments, it is proposed, in U.S. Pat. No. 4,123,208 to carry hot gas inwardly through radial bores in the nozzle assembly of the pot nozzle. These bores, however, can only carry very small quantities of gas inwards. The gas passes through the nozzle assembly, cools down and thereby heats the nozzle assembly. The effect obtained does not even approximate that obtained with a ring spinning nozzle.